During the methanization of biomass, fermenters are used in which a fermentation process runs, wherein methane-containing gas is produced by the fermentation of the biomass. A distinction is made between the so-called continuous process and the discontinuous process (batch process). In the latter, a percolate containing microorganisms is introduced into a fermenter, which percolates through the substrate (biomass) and is collected in a percolate container and is re-introduced into the fermenter, if necessary, until the biomass contained in the fermenter has been fermented. The fermented biomass is subsequently removed from the fermenter and must be purified, if necessary, and be utilized in the customary manner. To facilitate the recovery, it may be necessary that the fermented biomass is sanitized. This means that the biomass must be free of undesirable germs, bacteria, or other impurities, so that the fermentation residue can be recovered. The same is also applicable for the percolate which likewise contains corresponding bacteria and the like and must likewise be sanitized so as not to re-infect the already sanitized biomass in the fermenter during recycling.